The wide use of new energy leads to a rapid expansion of secondary battery market. The demand of secondary battery is sharply increased. A cheap, reliable, safe and long-life secondary battery is in urgent requirement no matter in electric vehicles, wind power, solar energy grid or regulation peak load. The current development of secondary battery mainly focuses on lithium-ion batteries, high temperature sodium-sulfur batteries, sodium nickel batteries and vanadium redox flow battery. These batteries have their advantages, such as lithium-ion batteries and sodium-sulfur battery with long life and high energy density, vanadium redox flow battery with theoretically unlimited life. However, no kind of batteries are able to satisfy cost, reliable, safe and long life requirements simultaneously. Conventional lithium-ion batteries are expensive and of safety risk; sodium-sulfur battery is expensive and of high manufacturing threshold; several technology bottlenecks of vanadium redox flow battery are currently unable to solve.
To solve this problem, many researchers turn to aqueous Li-ion battery, hoping to drastically reduce the cost and improve the safety of Li-ion batteries by using water-based electrolytes in place of organic electrolytes. In 1994, Jeff Dahn et al. presented an aqueous battery with LiMn2O4 as the cathode material, vanadium oxide such as LiV3O8 as the anode material, and a water solution of lithium salts as the electrolyte, but the toxicity of vanadium and poor stability of anode in water limit the development of this kind of battery. Up to now, all reported aqueous Li-ion batteries used the same principle as the Li-ion battery, based on an embedded type structure on both positive and negative electrodes, such as LiMn2O4/VO2, LiNi0.81Co0.19O2/LiV3O8, LiMn2O4/TiP2O7, LiMn2O4/LiTi2(PO4)3, and LiCoO2/LiV3O8.